The present invention generally relates to compositions for cleaning exposed lithographic plates, more particularly to improved emulsion compositions containing a specific combination of compounds which, when combined in admixture at particular relative concentrations are suitable for use as a finisher for imaged lithographic plates before they are run on the press while at the same time being useful as a superior plate cleaner and scratch remover for use within the press room in order to extend the overall useful life of an exposed and imaged printing plate as used during printing operations. Such compositions are multifunctional in that they provide in a single formulation the functions of removing ink from the image areas, desensitizing the non-image areas, reducing scumming and the appearance of scratches, and enhancing the oleophilic nature of the image areas, while at the same time not attacking the image areas.
Lithographic plate finishers of the emulsion type are well known from publications such as U.S. Pat. No. 4,162,920 as being useful for desensitizing the non-image areas of a lithographic printing plate while simultaneously conditioning the image areas of the plate to maintain the ink-receptivity thereof during storage, but such finishers typically require a subsequent plate roll-up of fifty cycles or more, whereby fifty or more preliminary printings that are of unacceptable quality must be completed before the plate is suitable for commercially acceptable printing. Also known heretofore are various other formulations that can be used as cleaners for plates that have become scummed, scratched or otherwise reduced in quality, which cleaners must be used in conjunction with a preliminary step of either first running the press dry, i.e. with the ink supply cut off, or first treating the plate with a non-aqueous solution, either of which preliminary steps must be then followed by a roll-up run on the order of thirty copies prior to actual application of the scratch remover. Both such prior art finishers and cleaners or scratch removers suffer the disadvantage of requiring wasteful lengthy roll-up, as well as the need to provide one formulation for finishing and another formulation for cleaning or scratch removal.
Accordingly, there has been brought about a need for a composition that is multifunctional in that it is suitable for finishing in association with plate making operations without requiring subsequent lengthy roll-up while at the same time also be a superior one-step cleaner and scratch remover for use in the press room that does not require a preliminary cleaning step necessitating wasteful plate cleaning pre-printing runs. By the present invention, a single composition is provided which is suitable for use as a plate finisher as well as a plate cleaner, either such use being one in which such superior results are accomplished without any extensive roll-up being required. The compositions of the present invention are emulsions of an oil phase and of an aqueous phase, the oil phase including an oleophilic acid, a highly penetrating solvent, and a surfactant for emulsifying the oil phase and the aqueous phase together. The aqueous phase includes a hydrophilic synthetic resin desensitizer, a gum desensitizer, a desensitizing agent of phosphoric acid and/or derivatives thereof, an alkaline nitrate salt, and water, the compounds of the emulsion being combined in particularly advantageous relative ratios to bring about the superior multifunctional attributes of this invention.
It is, therefore, a general object of the present invention to provide an improved cleaning composition.
Another object of this invention is to provide an improved cleaner and finisher that works effectively on virtually any lithographic printing plate, whether it be subtractive, additive, or wipe on.
Another object of this invention is to provide an improved multifunctional composition for treating lithographic printing plates while substantially reducing use of paper, ink, and the additional roll-up runs required to achieve commercially acceptable printing quality.
Another object of the present invention is to provide an improved emulsion composition that performs both as a finisher in the plate making room and also as a superior cleaner, conditioner or scratch remover in the press room.
Another object of the present invention is to provide a plate treating composition that is easy to apply and has a uniform, easy-to-handle consistency and that does not cake up when it passes through restricted areas such as necks of bottles.
Another object of this invention is an improved composition of various compounds in admixture that are compatible with each other at selected ratio ranges and which composition desensitizes and keeps the non-image areas of exposed plates hydrophilic on storage, renders the image areas more oleophilic, removes ink off of the plate on the press, and eliminates the printing of scratches on the non-image areas, including any minor scratches that are made during mounting and handling of the plates.
Another object of the present invention is to provide a superior water-in-oil emulsion having an aqueous phase for desensitizing non-image areas and an oil phase that removes oil receptive contamination from fingerprints, skin oils, hair oils, ink, or other sources.
Another object of this invention is to provide an improved composition that desensitizes and removes ink from non-image areas of exposed lithographic plates while restoring ink receptivity and insuring excellent fast image ink-up characteristics.
Another object of this invention is an improved plate cleaner that does not always require gumming after water washing it from the cleaned plate.
These and other objects of the present invention will be more apparent from the following further detailed description thereof.
The emulsion compositions in accordance with this invention include an oil phase of a highly penetrating solvent, an oleophilic acid, and an emulsifying surfactant, and an aqueous phase having a hydrophilic synthetic desensitizer, a gum desensitizer, phosphoric acid and/or derivatives thereof as a desensitizing agent, an alkaline nitrate salt, and water, optionally also including a polyol to assist in keeping the emulsion stable. An important aspect of this invention is the relative ratio of these various ingredients within the composition to provide a water-in-oil emulsion that accomplishes a multitude of functions without any significant interference among the various individual components thereof.
Emulsions according to this invention comprise the following quantities of compounds, each quantity being expressed as a weight percent based upon the total weight of the emulsion composition; the relative ratios of such compounds being within these values:
__________________________________________________________________________ COMPOUNDS RANGE PREFERRED RANGE MOST PREFERRED RANGE __________________________________________________________________________ (Oil Phase) Highly penetrating 7-23% 10-20% 13-17% solvent Oleophilic acid 0-15% 5-10% 7-8% Emulsifying 4-20% 5-15% 8-12% surfactant (Aqueous Phase) Hydrophilic synthetic 0.05-0.5% 0.1-0.3% 0.15-0.25% desensitizer Gum desensitizer 10-28% 12-25% 17-23% Phosphate desensi- 0.1%-2.5% 0.2-2% 0.5-1.5% -tizing agent Nitrate salt 0.2-7% 0.5-5% 1-3% Polyol emulsion 1-20% 2-10% 2.5-7.5% stabilizer (optional) Water 25-70% 32-45% 38-42% __________________________________________________________________________
The highly penetrating solvent included within the oil phase of the emulsion composition according to this invention must be one that has penetration properties that are of a strength that is capable of removing ink off of plates in the press room while at the same time not being overly penetrating so as to damage the image or non-image areas even when the formulation remains on the plate for extended periods of time such as when it is used as a finisher. Suitable solvents include mineral spirits, kerosene and lithotene (approximately 90 percent mineral spirits and 10 percent of an oleophilic resin), the preferred penetrating solvent being mineral spirits. It is preferred to maintain the mineral spirits content of the composition to as low a level as possible in order to minimize the tendency of the water-in-oil emulsion to break up during shelf storage. When an especially stable emulsion is desired the mineral spirits content should be not much greater than about 15 weight percent, based on the total weight of the composition.
An oleophilic acid is preferably included within the oil phase as a compound that is particularly advantageous in the emulsions according to this invention in ordr to make the image areas of the exposed and developed plates more oleophilic while at the same time being of assistance in stabilizing the emulsion. Especially preferred in this regard is oleic acid, although similar compounds such as stearic acid are suitable in some formulations according to this invention. If enhanced ink receptivity is not needed, the oleophilic acid can be omitted, provided the emulsifying surfactant has especially strong emulsifying properties, such as the isoactyl phenoxy polyethoxy ethanols.
Emulsifying surfactants within the oil phase must be adequate to assist in forming and to maintain the water-in-oil emulsion of this invention, preferably to the extent that phase separation is substantially eliminated for several months when the emulsion is on the shelf and stands undisturbed. Such surfactants may be either non-ionic, anionic or cationic, although cationic surfactants typically will not be used because they tend to disintegrate at the low pH values of these compositions. Non-ionic surfactants are generally preferred because they are themselves particularly stable within the very acidic environment of these compositions. Surfactants of this type include isoactyl phenoxy polyethoxy ethanols such as Triton N-57 of Rohm and Haas Company, non-ionic N-substituted fatty acid amides having polyoxythylene substituents, such as Ethomid 0/15 of Armour, a polyethylene glycol p-isoactylphenyl ether such as Igepal 630 of GAF Corporation, mixtures of sodium salts of sulfated fatty alcohols, including surfactants such as Duponal WAQ and Duponal ME of DuPont Corporation, long chain alcohol ethylene oxide condensates such as Merpol MC of DuPont Corporation, modified alkanol amides such as Richamide 6445 of The Richardson Company, and benzene sulfonate non-ionic surfactants. Especially preferred are the alkyl substituted benzene sulfonates, such as isopropylamine dodecyl benzene sulfonate, which have a superior ability to stabilize the emulsions according to this invention for extended periods of time, the use of other surfactants typically necessitating having to agitate the composition in order to reform a complete emulsion prior to use of the composition. The stability of the emulsion is further enhanced when the surfactant is not much less than 10 weight percent of the entire composition.
The hydrophilic synthetic resin desensitizer within the aqueous phase must, of course, be water soluble, typically being a hydrolyzed synthetic resin such as modified polyacrylamides, hydrolyzed styrene-maleic anhydride copolymers, hydrolyzed vinyl ether-maleic anhydride copolymers, and the like. Especially preferred is a modified polyacrylamide having a molecular weight of approximately 200,000 in which about 70% of the acrylamide groups thereof have been hydrolyzed to acrylic acid groups, such material being in the nature of a 70/30 copolymer of sodium acrylate and acrylamide, typically being provided as a 10% solution. Preferably, for solubility purposes, these hydrophilic synthetic desensitizers will be in the generally alkaline salt form.
Gum desensitizers useful within the aqueous phase of this invention may be either natural or synthetic, a readily available and most acceptable gum desensitizer being gum arabic, such as 14.degree. Be gum arabic. The gum desensitizer, when in admixture, with the hydrophilic synthetic desensitizer and the phosphoric desensitizing agent, brings about an especially advantageous desensitization of the plate on the press as well as maintenance of the hydrophilic character of the non-image areas while they are stored.
The phosphoric desensitizing agent, which works in admixture with the resin desensitizer and the gum desensitizer to swell the gum molecule in order, for example, to increase the water receptivity of the gum, also assists in maintaining the low pH of the overall emulsion system, typically between about 1.70 and 2.20 and preferably about 1.80. Such phosphoric desensitizing agent includes phosphoric acid or derivatives thereof, phosphoric acid itself typically being preferred.
Also included within the aqueous phase is an alkaline scratch eliminator, such as a nitrate salt, which is water soluble and functions to desensitize scratches on the non-image areas to the extent that the scratch will not accept the ink when the plate is printed. Especially useful scratch eliminators are magnesium nitrate, potassium nitrate, and the like.
An optional ingredient within the aqueous phase is a polyol emulsion stabilizer that is useful in reducing the need to agitate the composition prior to use in order to re-form the emulsion between the oil phase and the aqueous phase. Polyglycols have generally been found to be acceptable, and glycerine is the preferred polyol emulsion stabilizer. Carbowax materials are also useful in this regard, but are inferior.
The hydrophilic synthetic resin, the gum desensitizer and the phosphoric desensitizing agent, are especially advantageous in that they cooperate to provide a desensitizing composition that not only renders the non-image area oleophobic or hydrophilic during storage and serves to desensitize the plate on the press, but it also is compatible with the other components within the aqueous phase such as the nitrate salt scratch remover as well as with each of the compounds of the oil phase, especially the oleic acid oleophilic agent for the image area and the highly penetrating solvent press ink remover. It is this intercompatibility among the various ingredients of the emulsion composition at the specific relative ratios thereof that is an important aspect of this invention which enables the overall composition to be simultaneously multifunctional.
Compositions according to this invention clean ink from printing plates as used on the press without damaging the image areas, at the same time being suitable both to protect the non-image areas of the exposed and developed plate while in storage and to remove surface scratches therefrom. They desensitize the plate non-image background areas, protect the plate in storage, and condition the image areas to maintain them oleophilic, all while avoiding scumming on the plates. They have a viscosity on the order of about 68 to 77 centipoises, a specific gravity of about 0.998.+-.0.009, a tag closed cup flash point at about 175.degree.-185.degree. F., and have the appearance of a white emulsion.
In use, the compositions according to this invention are applied to an imaged lithographic printing plate. When used as a cleaner, they are applied to at least the affected areas of the plate with a damp sponge or a Webril wipe, after which they are left on the plate for at least 30 seconds, and then the surface is wiped clean or the plate is etched with a fountain solution, gum or water. At this stage, the plate is ready to be run on the press.
The following specific examples will more precisely illustrate this invention and teach the presently preferred procedures for practicing the same, as well as the advantages and improvements realized thereby.